In an active-matrix OLED panel, an image is formed by a large quantity of pixels arranged in a matrix. The brightness of each pixel is controlled by the data signal. In conventional arts, each pixel has a bias switch, a storage capacitor, a driving transistor and a light-emitting diode. When a scan line supplies a scan signal to a control terminal of the bias switch, the bias switch is turned on and the data line inputs a data signal via the bias switch to charge the storage capacitor. Then, the scan line stops supplying the scan signal so that the bias switch is turned-off. Therefore, the driving transistor is electrically separated from the data line. Hence, the gate voltage of the driving transistor stably maintains so that the data signal transmitted from the data line can be fed into the storage capacitor during a period of time. A driving current flowing through the light-emitting diode is determined by the voltage difference between the gate and the source of the driving transistor and the threshold voltage of the driving transistor. The light-emitting diode emits light according to the driving current.
One of the factors that affect the current flowing through the light emitting diode is the threshold voltage, and the threshold voltage usually varies because of the manufacturing variation. Besides the manufacturing variation, each of the light emitting diode decays in different rates according to the material properties. Therefore, when inputting same voltage signals, it may generate different driving currents to result in irregular brightness of the panel.
To overcome this problem, in the prior arts, there is the compensation circuit in the pixel to compensate the threshold voltage. Various compensation circuits have been applied to solve the above-mentioned problem, such as the disclosure in Taiwanese patent publication number I237913 and in U.S. Pat. No. 6,859,103. In these prior arts, one or more transistors, one or more current sources and/or changing the circuit design of the original components are added into the circuit design of the conventional pixel to compensate the threshold voltage. However, there are still some problems. By doing so it will have to increase the number of the components, and therefore make the design of the circuit of the pixel be more complicated, to reduce the aperture ratio and then to cause insufficient brightness of the panel. Besides, it will need to apply more complex control signals in the conventional pixel, such as to cause more difficulty in the quality control.